Patient supports are used in the medical field to support and (more importantly) position a patient relative to an item of medical equipment such as a radiotherapy device. The patient can lie in a prone or supine position on the support, which then lifts and moves the patient into the desired position relative to the item of medical equipment.
In the example of a radiotherapy device, this has an “isocentre” to and around which the therapeutic radiation is delivered. The device can be called upon to treat a lesion which may be in any of a range of positions within the patient, and thus the patient needs to be moved so as to place the lesion substantially at the isocentre. In addition, a radiotherapy device has an associated geometry by which the radiation is delivered, and the patient may have to be oriented relative to the device so as to take account of that geometry. For example, a gantry-arm linear accelerator radiotherapy device delivers radiation to the isocentre from a radiation head that rotates around the isocentre, thus meaning that the beam of radiation arrives from a range of directions within a vertical plane that includes the isocentre. It may be preferable or necessary to orient the patient so that sensitive organs are placed outside that plane, in order to minimise their dose. It will also be necessary to ensure that the rotation arc of the gantry is kept clear of obstructions.
The treatment position is therefore dictated by the medical apparatus and by the required prescription. It may bear little or no resemblance to a position that is accessible for the patient to get onto (or be placed onto) the support. Modern patient support systems therefore have the ability to adjust the patient position in all six degrees of freedom, both so that it can be moved from an accessible position to a treatment position, and also to allow adjustment of the treatment position as required. This requires a number of electrically-powered motors, and their associated control circuitry.
The control circuitry may be complex, and may be integrated with the control circuitry for the medical equipment so that the patient position can be controlled during treatment. Some items of medical equipment (such as, again, radiotherapy apparatus) include feedback systems that monitor the current location of a lesion within the patient and call for adjustments to the patient position so as to correct any positioning errors that are detected.
This presents a problem in the event of a power failure during treatment, in that the patient may be left in an inaccessible location, i.e. one from which they are unable (or unwilling) to dismount from the support, or one from which they cannot be lifted off by medical staff. This means that a backup power source must be provided; this is typically in the form of an uninterruptible power supply (“UPS”). These consist of a lead-acid battery together with control and sensing circuitry. When the primary power source is online, the lead-acid battery is charged. When the sensing circuitry detects a power loss, the lead-acid battery powers an inverter to provide AC power to replace the primary power source.
A result of this arrangement is that the battery is partially discharged each time the system is powered down, and then recharged again when the system is powered up. This repeated cycling of the battery means that it needs to be replaced every 12 months or so, to ensure it is always capable of holding enough charge to perform its function. As the backup battery has to power the entire table and the associated control system when the power fails, it is required to have a fairly large capacity. The consequence is that the running costs of the system are increased by having regularly to replace a substantial battery.
In recent times, so-called “ultracapacitors” have been proposed as alternatives to lead-acid batteries in UPS devices. However, their high cost and (relatively) lower capacities mean that their uses are largely confined to providing brief power in order to shut down IT equipment safely, or to “ride through” the period between the loss of a permanent primary power source and the arrival on-line of a secondary backup power source such as a local generator.